1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell (hereinafter abbreviated as PEFC) which uses as the fuel a reducing agent such as pure hydrogen or modified hydrogen obtained from methanol or fossil fuels and uses as the reaction gas such an oxidizing agent such as air or oxygen. In more particular, the invention relates to a PEFC which operates by unhumidified reaction gas.
2. Description of Related Art
A PEFC comprises a membrane/electrode assembly consisting essentially of a polymer electrolyte membrane and gas diffusion electrodes (hereinafter referred to as MEA), in which, as shown in FIG. 1, reactions represented by the formulas (1) and (2) take place respectively at the positive electrode 4 and the negative electrode 5. EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (1) EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (2)
When the above reactions take place, the protons generated at the negative electrode move to the positive electrode via the polymer electrolyte membrane 1. Since a polymer electrolyte does not show a high ionic conductivity unless it is in a sufficiently moistened condition, generally the reaction gas needs to be always humidified by the use of a humidifier or the like in order to prevent the electrolyte from drying. On the other hand, the gas diffusion layer 2 of the electrode needs to have a high gas permeability in order that a high current density can be obtained. Therefore, to prevent the blockage of the gas diffusion path, excessive moisture must be discharged to the outside of the MEA. For example, JP-A-6-295728 uses as the gas diffusion layer carbon paper formed essentially of carbon fiber made from polyacrylonitrile as the raw material and subjected to a water repellent treatment using fluoro-resin. In JP-A-7-134993, the diffusion layer of the fuel electrode is provided with a hydrophobicity gradient such that the hydrophobicity is increasingly low toward the catalyst layer side 3 and the diffusion layer of the positive electrode is provided with a hydrophobicity gradient such that the hydrophobicity is increasingly high toward the catalyst layer side. By adopting the above-mentioned structure, the lowering of the moisture content of the polymer electrolyte of the fuel electrode side is prevented and the so-called flooding, which is a phenomenon wherein the catalyst layer is wetted by water formed at the positive electrode side and results in the blockage of the gas diffusion path.
However, when the previous PEFC which operates by humidified gas, which is so designed as to discharge excessive water to the outside of the MEA, is operated by using an unhumidified gas, the polymer electrolyte membrane and the polymer electrolyte contained in the catalyst layer become dry and the movement of protons tends to take place with difficulty. Moreover, the polymer electrolyte in the catalyst layer undergoes contraction to decrease the area of the electrolyte covering the platinum catalyst, that is, the reaction area, resulting in the increase of the internal resistance of the PEFC; thus, a good characteristic property cannot be obtained. In the operation of a PEFC using unhumidified gas, on the other hand, the flooding caused by the water formed at the positive electrode hardly takes place unlike in the operation thereof using humidified gas, so that it is important to retain the water formed at the positive electrode inside the MEA without discharging the water to its outside. In a PEFC of the structure specified by JP-A-7-134993, also, when the cell is operated by using an unhumidified gas, the amount of water evaporated from the positive electrode is large and hence the internal resistance increases similarly.